Disk drives are well known in the computer art for providing secondary mass storage with random access. A disk drive essentially comprises one or more magnetic data storage disks rotating on a spindle by a spindle motor, within an enclosed housing. A magnetic transducer head is placed on an actuator arm and positioned very closely to each data storage surface by a slider suspended upon an air bearing. Servo information are typically written in servo sectors which interrupt data sectors or blocks on each disk. Servo information provide a servo control loop in the disk drive with head position information to enable a head positioner mechanism, such as a rotary voice coil motor, to move the actuator, and therefore the head, from track to track during random access track seeking operations, and to maintain the head in proper alignment with a track centerline during track following operations when user data is written to or read from the available data block storage areas of the disk surface.
A such, the servo control loop is used to control head positioning as the head is being moved transversely across tracks and to cause the head to remain over a particular data track as the disk spins. The servo loop controls the acceleration of the head which results from a force supplied by the electric motor on the actuator. The input to the servo system are readings of head position made by the head itself. The head position is determined from servo information written directly onto the disk by a servo writer as part of the manufacturing process. The servo information may include the track number as well as an indication of how far the recording head is from the track center line. That is, a certain number of bits of information on each track are reserved for indicating position. As the head passes over the indicators, the track over which the head is sitting is determined by the head itself and supplied to the servo system. The indicators are at regularly spaced locations. Thus the input to the servo is not continuous but is sampled.
A servo track writer is a manufacturing unit, typically stabilized on a large granite base or sufficient isolation, to minimize unwanted vibration and employ an encoder (e.g. laser interferometry) for position measurements. As shown by example in FIG. 1, a servo writer 10 typically includes a positioning system 12 for moving an arm 14 that carries a push-pin 16 which extends into a disk drive 17 for direct mechanical access to a head actuator arm 18 of the disk drive 17. The head actuator arm 18 carries a transducer head 20 for writing data to, and reading data from a data disk 22 in the disk drive. The servo writer 10 may also include a fixed head for writing a clock track onto a disk surface.
The servo writer 10 supplies power to the disk drive spindle motor (not shown) for rotating the disk 22 therein, and the positioning system 12 moves the actuator 18 and head 20 via the push-pin 16, across the raw disk 22 in the disk drive 17 to write track address and fine servo information at several specified locations called sectors for each track on the disk 22. As shown, the push-pin 16 extends through an opening in the disk drive 17 housing and mechanically contacts the actuator arm 18. This servowriter arm 14 which rotates about a shaft 24 coaxial with the axis of rotation of the actuator arm 18. The angular position of the shaft 24 is determined by a servo controlled motor in the servo writer positioning system 12 which uses an encoder or other interferometer for position feedback. In order to maintain the actuator arm 18 against the push-pin 16, a small bias current is applied to an actuator voice coil motor 26 of the disk drive 17, in a direction to oppose the movement of push-pin 16. This insures precision in locating the head in the servo track writing process.
FIG. 2 shows a block diagram of an example conventional feedback control scheme for the positioning system 12 of the servowriter 10 in FIG. 1. The positioning system 12 comprises a controller 28, a positioner 30 and an encoder 32. The encoder 32 senses and provides the rotational position of the actuator arm 18, to the controller 28 to control the positioner 30 for moving the push-pin 16, thereby injecting a force input to actuator assembly 18, to position the transducer head 20 for writing precise servo tracks on the disk 22 in the disk drive.
Another version of the servowriter 10 uses a non-contact optical push pin system, instead of mechanical push-pin 16 in FIG. 1. With the optical system, a small diffraction grating is placed on the actuator arm 18. The servo writer arm 14 includes an interferometric position sensor (e.g., encoder) that emits an optical beam into the hard disk drive and determines the position of the actuator arm 18 by monitoring the light diffracted from the grating. Closed loop servo control of the actuator arm 18 position with respect to the shaft 24 is achieved using this encoder to provide position feedback, and the internal disk drive voice coil motor 26 is controlled by the controller 28 and/or the positioner 30, to drive the actuator arm 18. The angular position of the servo writer arm shaft is still controlled by the positioning system of the servo writer.
In that case, the push-pin 16, is an optical push-pin, wherein the encoder senses relative motion/position between the arm 18 and the optical pushpin arm 14, and generates a feedback signal accordingly for the controller to actively control the VCM 26 to follow the optical push pin. Instead of a biasing force to the arm 18 in the mechanical case, there is a control signal to the VCM 26. The positioning system 12 uses the encoder feedback signal as the error between the laser head and arm 18 and determines errors in the arm 18 following the optical pushpin. The positioning system controls the VCM 26 such that the arm 18 follows the pushpin accurately, such that whenever the positioning system moves the optical pushpin (laser head) around, the VCM 26 follows it.
However, a disadvantage of above control schemes is that the only feedback position signal available is the rotational position of the push-pin via the encoder 32. The exact position of the head 20 over the disk 22 in the disk drive 17 is not known. The mechanical vibrations of the structures (actuator arm/suspensions, etc.) between the push-pin and the head 20 are not detected. Furthermore, the motion of the disk 22 which contributes to part of the head off-track in not detected. As such, corrections to head position to compensate for vibrations and head off-track position cannot be made, resulting in writing low quality servo tracks with the head, which may have to be re-written.
Another disadvantage of conventional servo writers is the lengthy servo writing time. In one conventional servo writing example, for a disk drive having two disks (four data storage surfaces) and requiring three servo-writer-controlled passes of the head over a single track during servo writing, total servo writing time might consume as much as 45 minutes or more. Thus, servo writing using conventional servo writers requires time penalties in the manufacturing process attributable to servo writer bottleneck.
There is therefore a need for a method and apparatus for servo writing which provides more precise position control of the servo writing head for improved servo writing quality, and reduces servo writing time and expense.